Having a crankshaft and two balancer shafts

Abstract

A balancer shaft of an engine includes a pair of journal portions, a first stage balance weight and a second stage balance weight. The first stage balance weight is formed between the journal portions and eccentrically arranged with respect to an axis. The second stage balance weight is formed on a center part of the first stage balance weight in the axial direction and projects outward from the first stage balance weight in a radial direction. The balancer shaft has a recess portion on the opposite side of the eccentricity direction.

What is claimed is: 
     
       1. An engine comprising a crankshaft and two balancer shafts, wherein:
 the two balancer shafts are arranged in such a manner that the axes of the two balancer shafts have a phase angle difference of substantially 180 degrees between the balancer shafts around the crankshaft axis, 
 a pair of journal portions is formed on opposite ends of each of the balancer shafts in the axial direction of the balancer shaft, the journal portions being rotatably supported by a crankcase, 
 a first stage balance weight is formed between the journal portions, the first stage balance weight being eccentrically arranged relative to an axis of the journal portions and having a recess portion on the opposite side of an eccentricity direction of the first stage balance weight, 
 a second stage balance weight is formed on a substantially center part of the first stage balance weight in the axial direction of the first stage balance weight, the second stage balance weight further projects outward from the first stage balance weight in the radial direction, and 
 the second stage balance weight is positioned between crank arms of the crankshaft when seen in a direction orthogonal to the axial direction of the balancer shaft, and one part of the second stage balance weight can pass between the crank arms. 
 
     
     
       2. The engine according to  claim 1 , wherein the first stage balance weight and the second stage balance weight are formed into a sector shape taking each balancer axis as a sector pivot when seen in the axial direction of the balancer shaft, and the sector shape has an open angle less than 180 degrees. 
     
     
       3. The engine according to  claim 1 , wherein the first stage balance weight and the second stage balance weight are formed into a sector shape taking each balancer axis as a sector pivot when seen in the axial direction of the balancer shaft, and the sector shape has an open angle of substantially 90 degrees. 
     
     
       4. The engine according to  claim 1 , wherein the first step weight portion is formed between the journal portions, and a driven gear and a governor mechanism are arranged near one of the journal portions. 
     
     
       5. The engine according to  claim 1 , wherein one of the balancer shafts is positioned on an upper side of the crankshaft axis and the other balancer shaft is positioned on a lower side of the crankshaft axis. 
     
     
       6. The engine according to  claim 5 , wherein the other balancer shaft, which is arranged below the crankshaft, has an oil pull surface. 
     
     
       7. An engine comprising:
 a crankcase; 
 a crankshaft having a crankshaft axis, the crankshaft being rotatably supported by crankcase; and 
 a first balancer shaft having a first balancer shaft axis; 
 a second balancer shaft having a second balancer shaft axis, the first and second balancer shafts being arranged in such a manner that the axes of the first and second balancer shafts have a phase angle difference of substantially 180 degrees between the first and second balancer shafts around the crankshaft axis, and 
 a camshaft arranged substantially in parallel to the crankshaft, the first balancer shaft and the second balancer shaft, wherein: 
 ends of the first balancer shaft define a pair of journal portions rotatably supported by the crankcase, 
 ends of the second balancer shaft define a pair of journal portions rotatably supported by the crankcase, 
 each of the first and second balancer shafts having a first stage balance weight formed between the journal portions, the first stage balance weight being eccentrically arranged relative to an axis of the journal portions and defining a recess portion on an opposite side of an eccentricity direction of the first stage balance weight, 
 a second stage balance weight is formed on a substantially center part of the first stage balance weight in the axial direction of the first stage balance weight, the second stage balance weight projecting outward from the first stage balance weight in a radial direction, and 
 the second stage balance weight can be positioned between crank arms of the crankshaft when seen in a direction orthogonal to the axial direction of the respective balancer shaft so that part of the second stage balance weight can pass between the crank arms. 
 
     
     
       8. The engine according to  claim 7 , wherein the first stage balance weight and the second stage balance weight are formed into a sector shape taking each balancer axis as a sector pivot when seen in the axial direction of the balancer shaft, and the sector shape forms an angle less than 180 degrees. 
     
     
       9. The engine according to  claim 7 , wherein the first stage balance weight and the second stage balance weight are formed into a sector shape taking each balancer axis as a sector pivot when seen in the axial direction of the balancer shaft, and the sector shape forms an angle of substantially 90 degrees. 
     
     
       10. The engine according to  claim 7 , wherein a driven gear and a governor mechanism are arranged near one of the journal portions of the first balancer shaft. 
     
     
       11. The engine according to  claim 7 , wherein the first balancer shaft is positioned above the crankshaft axis and the second balancer shaft is positioned below the crankshaft axis. 
     
     
       12. The engine according to  claim 11 , wherein the second balancer shaft includes an oil pull surface. 
     
     
       13. The engine according to  claim 12 , wherein the axes of the crankshaft, the first balancer shaft, and the second balancer shaft are positioned on the same line.